Both natural and synthetic rubber latexes are emulsions of rubber in water. The emulsions are stabilized with proteins or surface-active agents. The median diameter of rubber particles average varies from 0.5 to 2.0 micron. It may be as low as 0.1 micron for some synthetic latexes and tends to be nearly 1 micron for natural latex. The term “latex” is applied to synthetic as well as natural products, and stems from the botanical name of the milky fluid that is derived from rubber trees and which is the source of natural rubber. The term “serum” is used for the aqueous medium.
Processes for producing artificial latexes have been known for many years. For instance, the process in U.S. Pat. No. 2,595,797 provides the following steps:                1. preparing a solution of a rubber in a water-insoluble volatile organic solvent in a concentration sufficient for emulsification;        2. introducing the solution under pressure into water containing a surface active agent;        3. adding an antifoamant (e.g., polysilicone oil) and agitating the mixture until emulsion is obtained;        4. removing the solvent by flashing (whilst avoiding excessive foaming), and        5. concentrating the solids content of the latex by allowing it to stand for 24 hours and removing some of the serum.        
In U.S. Pat. No. 2,799,662 a similar process is described. The method consists of a number of integrated steps which include the dissolving of the dry polymeric material in a suitable selected solvent, dispersing the thus prepared polymer solution into a carefully selected and adjusted water-emulsifier system and, finally, stripping out the solvent to leave the polymer dispersion as artificial latex. According to this reference, it is highly desirable to have present two emulsifiers present, one of the type which is hydrocarbon soluble (e.g., alkali metal petroleum sulfonates having 20 to 21 carbon atoms arranged in an alkyl-aryl structure) and one of the water soluble type (e.g., alkali metal sulphate derivatives of higher alcohols). Emulsification of the polymer solvent mixture is accomplished under conditions preventing flashing of the solvent.
The problem of emulsion stability when stripping off the solvent is addressed in U.S. Pat. No. 2,871,137, which provides a method for preparing emulsifying agents based on the hydrocarbon polymers that are emulsified.
A method for preparing stable emulsions of polymeric or resinous materials is furthermore described in U.S. Pat. No. 2,947,715. This is accomplished by dissolving the rubber or resin in a suitable solvent, adding a creaming agent to the polymer solution during emulsification, and creaming the resultant latex prior to removal of the solvent, removing the solvent and then again creaming the solvent-free latex.
In U.S. Pat. No. 2,955,094, ortho-phosphoric acid and organic sulphate salts are used as emulsifiers. As indicated in this reference, experience has shown that latexes are relatively unstable and tend to coagulate when subjected to mechanical stress. Mechanical instability may be brought about by the simple movement of an agitator stirring the colloid. Maintenance costs are increased because the equipment becomes coated with the coagulated rubber and furthermore, an appreciable quantity of the rubber is lost. Another type of instability encountered with polymer latexes is that they oil-out and develop coagulum during the solvent stripping step.
U.S. Pat. No. 3,250,737 sets out to produce concentrated latexes of synthetic elastomers from organic solutions thereof in a manner that is both rapid, efficient and economical. This is accomplished by mixing a solution of a synthetic elastomer in an organic solvent, water and an emulsifying agent, homogenizing the mixture at least until the resulting emulsion is stable, stripping the organic solvent at elevated temperatures and pressures below conditions at which water boils, centrifuging the resulting dilute aqueous latex, recovering and recycling the aqueous serum from the said centrifuging step and recovering the concentrated latex. This reference concentrates on the steps of flashing and centrifuging, it is immaterial how the hydrocarbon solution is made.
Of particular interest are continuous processes for preparing artificial latexes, comprising a continuous emulsification step. Patent references describing continuous emulsification steps are manifold; examples thereof include: U.S. Pat. Nos. 3,622,127; 4,344,859; 4,243,566; 3,879,327; 3,862,078; 3,892,698; 3,879,326; 3,839,258; 3,815,655; 3,719,572; 3,652,482; 3,644,263; 3,294,719; 3,310,515; 3,277,037; 3,261,792; 3,249,566; 3,250,737 and 2955094, as well as GB1384591; FR2172455 and NL7212608. These references describe the need for ultradispersing equipments or homogenizers of various sorts.
A further reference of interest on this process step is EP0863173, which relates to (claim 1):
A process for preparing stable polymer dispersions with polymer particle sizes of 0.1 to 10 μm [d50 determined in an ultracentrifuge], characterized in that a water-in-oil emulsion, comprising a polymer dissolved in an organic solvent which is immiscible with water (organic phase) and an aqueous phase, wherein:
the viscosity of the organic phase is 1.0 to 20,000 mPa·s (measured at 25° C.);
the surface tension between organic and aqueous phase is 0.01 to 30 mN/m;
the particle size of the water emulsified in the organic phase is 0.2 to 50 mm and the ratio by volume of organic phase to aqueous phase is in the range of from 80:20 to 20:80,
is subjected to a shear process at a shear power of 1×103 to 1×108 Watts/cm3, and wherein the water-in-oil emulsion is converted into an oil-in-water emulsion. This process is specifically dedicated to dispersions having very small particle sizes. Moreover, the equipment to be used in this process is very hard to scale-up to commercial size.
The particle size and particle size distribution of the oil-in-water emulsion made at the emulsification step is important for reasons of latex stability, but also for subsequent use of the latex by companies preparing gloves, condoms and the like. In addition, it is at this stage that the particle size and the particle size distribution of the final aqueous emulsion are determined. On the other hand, it has been found that the molecular weight of isoprene rubber is reduced in this step, by mechanical degradation, which adversely affects the properties of the articles prepared there from. Thus, there is an interest in an improved (continuous) emulsification step that has little or no adverse impact on the molecular weight, whereas producing an oil-in-water emulsion that is both stable and composed of particles of the right size and size distribution.
For IR latex, the right particle size in the artificial latex is from about 0.5 to about 2 μm, whereas these particles will be larger in the preceding oil-in-water emulsion, due to the presence of solvent within the emulsified rubber/solvent particles.
It is therefore an object of the present invention to provide an improved process for the preparation of an oil-in-water emulsion to be used in the preparation of an artificial latex.